This invention relates to an aspherical lens element for use in an object lens system of a rewritable optical disk device.
In a rewritable or erasable optical disk device of the type described, a laser diode or a semiconductor laser is used to write information on an optical disk. In this event, the semiconductor laser generates a laser beam which has different outputs or amplitudes on a write-in operation or on a readout operation. In other words, the outputs of the laser beam are widely varied between the write-in operation and the readout operation. When the outputs of the laser beam are widely varied, a distribution of oscillation wavelengths is also varied during each of the write-in operation and the readout operation.
In the meanwhile, a double-aspherical plastic lens element is disclosed by Arai et al in U.S. Pat. No. 4,449,792 and is widely used in a compact disk player in the form of a single lens element. However, such a double-aspherical plastic lens element is not pertinent for the optical disk device. This is because use of such a double-aspherical plastic lens element results in a wide variation of a focal point which comes from a variation of the oscillation wavelengths. Consequently, it takes a long time to control the focal point by the use of an actuator. This makes it difficult to carry out the write-in and the readout operations at a high speed because the outputs of the semiconductor laser can not be quickly switched from one to another.
On the other hand, consideration might be made about forming an achromatic optical system which comprises a plurality of object lens elements, so as to solve a problem resulting from a variation of the distribution of the oscillation wavelengths. However, such an achromatic optical system is expensive due to an increase of the lens elements and a difficulty of assembly of the lens elements. In addition, the achromatic optical system is heavy in weight and therefore makes a high speed operation difficult on controlling the focal point. Therefore, the achromatic optical system is not practically effective to solve the above-mentioned problem.
Alternatively, proposals have been offered about using a double-aspherical lens element which is formed by a low dispersion glass material and which may be referred to as a double-aspherical glass lens element which has an entrance or first surface and an exit or second surface. The double-aspherical glass lens element can be readily manufactured by glass molding. When such a double-aspherical glass lens element is used as the object lens element of the optical disk device mentioned above, it is possible to extremely reduce or decrease a variation of a focal point due to a variation of oscillation wavelengths in comparison with the double-aspherical plastic lens element mentioned above. This shows that the double-aspherical glass lens element can control the focal point within a very short time and can thereby carry out the write-in and the readout operations at a high speed.
In addition, inasmuch as the low dispersion glass material itself has excellent heat resistance, humidity durability, and temperature characteristic.
From this fact, it is readily understood that the double-aspherical glass lens element of the low dispersion glass material is very suitable for the object lens element of the rewritable optical disk device.
It is mentioned here that, when a refraction index is measured by the use of a d-line, such low dispersion glass generally has a low refractive index n which is lower than 1.55. Therefore, it is necessary to make a radius of curvature of the entrance of the refraction surface extremely small and to make a thickness of a lens element thick, so as to accomplish a desired focal length by the use of the low dispersion glass.
However, decentration between the first and the second surfaces of each lens element becomes large as the radius of curvature and the thickness are small and thick, respectively, and results in serious aberrations, such as comae. Consequently, it is very difficult to manufacture or produce such a double-aspherical glass lens element having the desired focal length. In addition, the double-aspherical glass lens element becomes heavy in comparison with the double-aspherical plastic lens element and with an increase of the thickness of the double-aspherical glass lens element. As a result, it is difficult to carry out tracking control and focal control at a high speed.